Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:    AP access point    ATIM announcement traffic indication message    MAC medium access control    IBSS independent basic service set    MSDU MAC service data unit    MPDU MAC protocol data unit    MP mesh point    MESH DTIM MESH delivery traffic indication message    SAP service access point    STA station    UWB ultra-wideband    WLAN wireless local area network
Two publications of interest to the ensuing description include:    (A) Standard ECMA-368, 1st Edition/December 2005, High Rate Ultra Wideband PHY and MAC Standard; and    (B) IEEE P802.11s™/D1.03, Draft STANDARD for Information Technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: ESS Mesh Networking (April 2007).
As defined in the ECMA-368 Standard a beacon group (BG) is a set of devices from which a device receives beacons that identify the same beacon period start time (BPST) as the device. A beacon period (BP) is the period of time declared by a device during which it sends or listens for beacons. A beacon period start time (BPST) is the start of the beacon period. Coordination of devices within radio range is achieved by the exchange of beacon frames. Periodic beacon transmission enables device discovery, supports dynamic network organization, and provides support for mobility. Beacons provide the basic timing for the network and carry reservation and scheduling information for accessing the medium.
As described in the IEEE P802.11s™ Draft Standard, in section 5.2.9.1 “Introduction to mesh”, in WLAN deployments without mesh services, stations (STAs) must associate with an AP in order to gain access to the network. These STAs are dependent on the AP with which they are associated to communicate. An example of a non-mesh WLAN deployment model and device classes are illustrated herein in FIG. 1, which reproduces Figure s1 of the IEEE P802.11s™ Draft Standard.
Many WLAN devices can benefit from support for more flexible wireless connectivity. Functionally, the DS of an AP can be replaced with wireless links or multi-hop paths between multiple APs. Devices traditionally categorized as clients can benefit from the ability to establish peer-to-peer wireless links with neighboring clients and APs in a mesh network.
An example mesh is illustrated in FIG. 2, which reproduces Figure s2 of the IEEE P802.11s™ Draft Standard. Mesh points (MPs) are entities that support mesh services, i.e., they participate in the formation and operation of the mesh network. An MP may be collocated with one or more other entities (e.g., AP, portal, etc.). The configuration of an MP that is collocated with an Access Point is referred to as a Mesh Access Point (MAP). Such a configuration allows a single entity to logically provide both mesh functionalities and AP functionalities simultaneously. STAs associate with APs to gain access to the network. Only MPs participate in mesh functionalities such as path selection and forwarding, etc. Mesh portals (MPPs) interface the network to other IEEE 802 LAN segments.
As is stated in section 5.2.9.2, “Mesh network model”, of the IEEE P802.11s™ Draft Standard, a mesh network is an IEEE 802 LAN comprised of IEEE 802.11 links and control elements to forward frames among the network members. Effectively, this means that a mesh network appears functionally equivalent to a broadcast Ethernet from the perspective of other networks and higher layer protocols. Thus, it normally appears as if all MPs in a mesh are directly connected to the link layer. This functionality is transparent to higher layer protocols. Reference in this regard can be made to FIG. 3A, which reproduces Figure s-3 of the IEEE P802.11s™ Draft Standard. It should be noted that while this Figure shows the forwarding of data over multiple hops, there may also be direct data transfer over a single hop, such as is shown in FIG. 3B, wherein the source and destination of the MSDUs are within a one-hop neighborhood, and where no forwarding, routing or link metric need be used.
An ATIM period, which may also be referred to without loss of generality as an “awake window”, is the time period after target beacon transmission time (TBTT) during which frames delivery initiation messages may be transmitted. An ATIM frame is used after a beacon frame to initiate frames transmission. An IBSS mode has beacons, similarly as in infrastructure mode. IBSS beacon transmission and infrastructure beacon transmission rules differ. In infrastructure beaconing one AP transmits one beacon, while in IBSS multiple stations compete for a beacon transmission opportunity, and a given station either receives a beacon from another station in the same IBSS network or transmits a beacon. Reference in this regard may be had to IEEE 802.11-1999 reaff 2003, sections 11.1.2.1 and 11.1.2.2.
802.11s specifies that the ATIM period (awake window) is used after the infrastructure or IBSS beacon, if the MP operates in power save mode.
The packet sets the synchronization of the group and announces that messages are waiting to be delivered. Stations in power save mode wake up periodically to listen for ATIM packets in ad hoc (IBSS) networks, just as they do for Beacon packets in infrastructure (BSS or ESS) networks.
A power-consumption problem exists in the foregoing and other types of wireless networks that is related to a need to minimize the activity time of a MP, such as the periodic media listening time of the MP. The receiving of the beacons of other MPs can consume a significant amount of power, especially if the beacons are transmitted separately, each at its own appointed time. The power consumption problem is particularly of concern in battery powered MPs.
As currently specified the MP is expected to receive all peer MPs beacons, i.e., all MPs to which the local MP has a link, and remain active during its own beacon plus the ATIM period time.
The above-referenced ECMA-368 Standard provides two power management modes in which a device can operate: active and hibernation. Devices in active mode transmit and receive beacons in every superframe. Devices in hibernation mode hibernate for multiple superframes and do not transmit or receive in those superframes. In addition, the ECMA-368 Standard provides facilities to support devices that sleep for portions of each superframe in order to save power. To coordinate with neighbors, a device indicates its intention to hibernate by including a Hibernation Mode IE in its beacon. The Hibernation Mode IE specifies the number of superframes in which the device will sleep and will not send or receive beacons or any other frames. Section 17.13, “Power Management Mechanisms”, of the ECMA-368 Standard is incorporated by reference herein in its entirety.
Three representative publications that generally address power management in IEEE 802.11 networks include:    “Optimal ATIM size for 802.11 networks in ad hoc mode”, X. Gao et al., DoCoMo Communications Lab USA (2006);    US Patent Publication No.: 2007/0133448, Jun. 14, 2007, “Method and Apparatus for Optimal ATIM Size Setup for 802.11 Networks in an Ad Hoc Mode”, X. Gao et al.; and US Patent Publication No.: 2006/0251004, Nov. 9, 2006, “Power Management in an IEEE 802.11 IBSS WLAN Using an Adaptive ATIM Window”, Z. Zhong et al.